1. Field of the Invention
This present invention relates to ethylenically unsaturated polyurethanes containing allophanate groups, to unsaturated, isocyanate group-containing precursors of these polyurethanes and to the use of the polyurethanes as binders in one-component coating compositions.
2. Description of the Prior Art
Aliphatic polyurethanes that contain acryloyl groups are known and described in U.S. Pat. Nos. 5,125,432, 5,136,009 and 5,300,615 and in DE-A 4,404,616. U.S. Pat. No. 5,125,432 is directed to the preparation of liquid urethane acrylates from monomeric diisocyanates, such as HDI or IPDI. To avoid the formation of solid products, it is necessary react the monomeric diisocyanate with a mixture of hydroxy acrylates, a mono-functional alcohol containing ester groups (such as trimethylolpropane (TMP) diacetate or diacrylate, which may optionally be alkoxylated) and a saturated, polyhydric alcohol such as TMP.
U.S. Pat. No. 5,136,009 is directed to the preparation of urethane acrylates from trimethyl-HDI by reacting this diisocyanate with a mixture of hydroxy acrylates and saturated, polyhydric alcohols such as TMP. U.S. Pat. No. 5,300,615 discloses that the urethane acrylates from U.S. Pat. No. 5,125,432 become turbid at less than 10.degree. C. This problem is overcome by reacting a mixture of HDI and IPDI with a mixture of hydroxy acrylates, an alkoxylated monofunctional alcohol containing ester groups (such as TMP diacetate or diacrylate, which is alkoxylated), a branched, saturated, mono or dihydric alcohol and optionally a linear, saturated mono or dihydric alcohol.
DE-A 4,404,616 is directed to a coating composition for polycarbonate which contains the reaction product of a low viscosity polyisocyanate, such as a low viscosity HDI trimer, with a hydroxy acrylate. The composition also contains a bis-acrylate as a reactive diluent.
A disadvantage of the compositions described in the U.S. patents is that they are too viscous for use in solvent-free, one-component coating compositions. Attempts to lower the viscosity by directly reacting monomeric diisocyanates, such as HDI or IPDI, with hydroxy acrylates results in the formation of solid products. Attempts to use low viscosity polyisocyanates, such the HDI trimer described in the German publication, also results in the formation of the solid products in the absence of the reactive diluent.
Accordingly, it is an object of the present invention to provide ethylenically unsaturated polyurethanes which do not suffer from the disadvantages of the prior art. It is an additional object of the present invention to provide ethylenically unsaturated polyurethanes that are suitable for use as binders for one-component coating compositions and have an acceptably low viscosity without the need for organic solvents and reactive diluents that are regulated environmentally objectionable and regulated by the government.
These objects may be achieved with the ethylenically unsaturated polyurethanes according to the present invention, which are described in detail hereinafter. The polyurethanes are derived from ethylenically unsaturated polyisocyanates containing allophanate groups.
Polyisocyanates containing allophanate groups have been described in U.S. Pat. Nos. 4,160,080 and 4,810,820 and in British Patent 994,890. Although ethylenically unsaturated alcohols are included in lengthy lists of suitable alcohol starting materials, there is no indication that these unsaturated alcohols could be used to prepare ethylenically unsaturated polyurethanes that satisfy the objectives of the present invention.
U.S. Pat. Nos. 5,459,214 and 5,580,947 are directed to ethylenically unsaturated isocyanates which may be used as binders in one-component coating compositions. However, these polyisocyanates do not contain allophanate groups as required by the present invention. The allophanate polyisocyanates described in U.S. Pat. Nos. 5,124,427, 5,208,334 and 5,235,018 and in EP-A-0,566,037 are not based on unsaturated alcohols and are recommended as crosslinking agents for two-component coating compositions.